Induction of broadly neutralizing antibodies (bNAb) against diverse isolates of human immunodeficiency virus (HIV) remains an unmet goal in AIDS vaccines research. Understanding the nature of the epitopes recognized by bNAb and the mechanisms by which such antibodies are generated are therefore of critical importance. Recent studies from Dr. L. Morris' lab in South Africa have identified a serum from a patient (CAP256), who possesses potent and broadly-reactive neutralizing activities for subtype C isolates. The key determinant of this reactivity maps to the V1V2 region and is likely to consist of highly discontinuous or quaternary structures. Similar structures may be targeted by novel monoclonal antibodies (MAb) characterized by several groups that also possess very potent neutralizing activities. The overall objective of this proposal is to develop macaque models to study the nature of the bNAb responses against the quaternary neutralizing epitopes (QNE) and factors that contribute to the generation of these antibodies. We hypothesize that antigenic stimulation in the context of infection by a replicating virus expressing QNE favors the generation of responses against these epitopes, and that repeated exposure by superinfection by viruses bearing related forms of such epitopes reinforces these responses: To test these hypotheses, we propose to adapt existing chimeric simian-human immunodeficiency viruses (SHIVs) to express the V1V2 determinants of conserved QNEs and to test their infectivity and pathogenicity in macaques. The availability of such models would allow us to compare the immunogenicity of viruses expressing various forms of QNEs and to isolate Mab specific for these epitopes. The Specific Aims of this Project are: (1) To determine the in vivo infectivity and immunogenicity of chimeric SHIV bearing the QNE of CAP256 and related isolates; (2) To examine superinfection by different QNE-bearing chimeras as an approach to enhance bNAb responses; and (3) To examine the evolution of envelope sequences in animals that develop differential responses against the QNE after SHIV infection. Results from these studies will provide further insight on the nature of the bNAb responses against HIV-1 and the design of novel immunogens to generate such responses.